I've killed a titan using HoS with one sniper shot. I've almost killed another (possibly same) with four shots from my Fulcrum. It's possible to kill them, but only if they have health regeneration perk and they haven't tossed any hammers yet. If they are using the armor perk, forget it. Also, I think the number of hammers tossed should be maxed at 4. That's the max a Hunter gets with golden gun using and exotic helmet perk.